1. Field of the Invention
The present invention relates to a compound semiconductor pellet, and also to a method for dicing a compound semiconductor wafer.
2. Description of the Related Art
A compound semiconductor wafer formed of a single crystal is diced along a cleavage plane since along this plane the single crystal easily splits. For example, a GaAs single crystal, one of III-V compound semiconductor, has a cleavage plane expressed as {011}, so that it is diced along the {011} plane in the direction of [011].
In the present specification, the crystal plane and the crystal orientation are expressed on the basis of the Miller indices. In the case where a cubic crystal has identical planes, such as (100), (100), (010), (010), (001) and (001), these identical planes will be generically represented by a notation using braces, such as {100}. The direction in which a crystal axis extends (i.e., a crystal orientation) will be denoted by a notation using the symbols "&lt;" and "&gt;", and equivalent crystal orientations, such as &lt;001&gt;and &lt;001&gt;will be generically represented by a notation using brackets, such as [001].
FIG. 1 is a plan view of a GaAs wafer. This GaAs wafer is a well-known type wherein the major surface on which elements are to be formed is a (100) plane.
A is shown in FIG. 1, the cleavage plane of the GaAs wafer 100 (which is a single crystal) is a {011}. plane, so that the GaAs wafer 100 is diced in the [011]direction, as is indicated by arrow A. Reference numeral 104 in FIG. 1 denotes dicing lines.
The wafer 100 has an orientation flat 102. This orientation flat 102 allows easy recognition of the posture of the wafer 100 in the semiconductor device-manufacturing process, or allows easy recognition of the cleavage plane of the wafer 100. The orientation flat 102 is formed by cutting the wafer 100 along the cleavage plane, i.e., in the [011] direction, such that a [011] plane is exposed.
FIG. 2 is a schematic perspective view of a GaAs pellet obtained by the dicing method mentioned above.
As is shown in FIG. 2, the major surface 106 of the GaAs pellet 108 is a (100) plane, and semiconductor elements 105 are formed on the major surface. The side surfaces 110 of the GaAs pellet 108 are exposed cleavage planes (011), (011), (01 1) and (011), respectively. That is, all side faces 110 of the GaAs pellet 104 are {011} planes if a (100) plane is selected as the major surface of the pellet 104.
The reason for selecting the cleavage plane as a dicing plane is that the semiconductor wafer easily cracks along the cleavage plane if only the groove is formed in the wafer by use of a blade or the like. It should be noted, however, that if the groove is formed in the [011] direction, a large kerf width will be produced at the time of dicing.
FIG. 3 is an enlarged view of the portion circled in FIG. 1 and illustrates details of a dicing line 104 and its neighboring regions.
As is shown in FIG. 3, the kerf width 112 will be large if the groove is formed in the [011] direction. In addition, a coarse surface will be produced at the time of dicing the wafer, as indicated by 114 in FIG. 3. If the surface is greatly roughened, not only the reliability of a semiconductor element formed inside the pellet 106 but also the outward appearance of the pellet 106 is adversely affected. In some cases, the pellet 106 itself may be cut at the time of dicing, resulting in damage to the element formed inside the pellet 106.
In the case of a III-V compound semiconductor single crystal, the atoms join in two ways: covalent bonding which occurs through electron clouds expanding in four directions; and ionic bonding which is based on Coulomb force. The reason that the Coulomb force is generated is that the III-group atoms and the V-group atoms differ in electronegativity. More specifically, the III-group atoms tend to emit three electrons and become a cation having a valence of +3, while the V-group atoms tend to receive three electrons and become an anion having a valence of -3.
The atoms of a single silicon or germanium crystal join by the covalent bonding alone. In contrast, the atoms of the III-V compound semiconductor single crystal join by both the covalent bonding and ionic bonding, as mentioned above. In connection with the III-V compound semiconductor single crystal, it should be also noted that the force of the ionic bonding is greater than that of the covalent bonding, so that the direction of the interatomic bonding force is determined mainly by the direction of the ionic bonding.
As may be understood from the above, the III-V compound semiconductor single crystal differs from the single silicon or germanium crystal, in light of the easiness of cleavage, the direction of cleavage, etc. The differences will be described in more detail below.
In the base of the III-V compound semiconductor single crystal, a {111} planes is totally occupied with either III-group atoms or V-group atoms. In addition, since the III-group atoms of one {111} plane and the V-group atoms of another 111 plane are very close to each other, the Coulomb force acting between the two {111} planes is so intense as to provide the strongest possible bonding. Unlike the {111} planes, the {011} planes are occupied with III-group atoms and V-group atoms. Since the {011} planes are electrically neutral and the Coulomb force weak, they are used as cleavage planes.
Let it be assumed that the groove is formed in the III-V compound semiconductor single crystal by moving a blade along a {011} plane (i.e., a cleavage plane) in the [011] direction. In this case, the movement of the blade may be affected by a {111} plane (i.e., a plane which provides the strongest possible bonding) since the {111} plane is 45.degree. slanted with reference to the depth direction of the crystal. That is, the blade may slide along the {111} plane when the groove is formed.
In practice, the {111} planes are hard to expose in a satisfactory manner. As is indicated in FIG. 3, there may be a certain kerf width 112, and the exposed surfaces may be coarse.
As long as the groove formed by use of the blade is not deep, the above problems can be solved to a certain extent, even if the blade is slid in the [011] direction. If the groove is formed by use of a diamond scriber, the above problems can be almost solved.
However, if the groove formed by the blade is shallow or if it is formed by use of a diamond scriber, the thickness of that portion of the crystal which should be divided by cracking will increase. As a result, the following problems occur:
(1) It is likely that crack-dust will be produced in large quantities at the time of cracking the GaAs wafer. If crack-dust is produced, it is hard to remove, since the GaAs wafer is a semi-insulating material and is electrified due to the static electricity generated at the time of cracking. If such crackdust is left on the pellet, wire bonding cannot be satisfactorily performed with respect to the pellet. In addition, the reliability of the semiconductor device incorporating the pellet may be adversely affected.
(2) The direction of the dicing line has to be accurately the same as the direction of cleavage. If not, the pellets obtained by dicing the GaAs wafer cannot be easily separated from one another. This means that the wafer has to be set accurately at the right position in the photolithography step of the manufacture of a semiconductor element. If the wafer is not accurately set, the mass production of pellets is adversely affected.
(3) To crack the GaAs wafer, a great load is required, so that the GaAs wafer may be mechanically damaged. If the GaAs wafer is mechanically damaged, a pellet is stressed, resulting in deterioration in the reliability of the pellet.